In the building and construction trade, considerable effort has been expended in improving and devising techniques for curing concrete articles such as blocks and similar structural units. As used in the art, the term "curing" as applied to concrete articles refers to the period between the molding operation and the time when the articles are strong enough to be used and during which hydration or hardening of the cement takes place through chemical reaction between the cement and water. Regardless of the curing technique employed, the curing parameters of time, temperature and moisture are the most important and must be suitably balanced so as to maintain the concrete in a moist condition until the cement has properly hydrated or hardened. Of course, these parameters depend to some extent upon the selection of the concrete aggregates, the amount of cement used and the desired resulting properties of the concrete articles as is well known to the skilled artisan.
The oldest curing technique is natural curing wherein the concrete articles are subjected to atmospheric conditions existing at the time the articles are made without special heating or wetting. The major drawbacks of natural curing are that the moisture needed for hydration of the cement is left to chance depending on weather conditions and that the curing process requires several weeks to 28 days. As a consequence and in view of the ever-increasing demand for concrete articles, natural curing has for the most part been replaced by more accelerated curing techniques.
One technique currently in use is low pressure steam curing. In this method and usually after the unavoidable time it takes to load the kiln, a sufficient quantity of steam is injected directly into the kiln to heat the internal kiln atmosphere to the desired temperature and at the same time maintain a high degree of saturation thereby providing the combination of heat and moisture which accelerates the hydration and hardening of the cement. Normally, saturated steam at atmospheric pressure is used to attain temperatures on the order of 140.degree.-180.degree. F or higher. In many instances, additional heat and moisture are added to the kiln atmosphere by burning a gas and recirculating the products of combustion, with or without the addition of moisture.
Low pressure steam curing is disadvantageous in that it is difficult to achieve a uniform kiln atmosphere at these relatively low steam pressures and therefore products of non-uniform quality are produced. One drawback is that during the time it takes to fully charge the kiln, i.e., on the order of 3 hours, the concrete articles first charged into the kiln will be cured at a different rate than the articles last charged in the kiln. Another drawback is that the total energy input in such a system is high due to the fact that considerable energy must be expended in generating steam and in supplying additional heat energy by burning fuel. Moreover, when burning fuel, the products of combustion, such as carbon dioxide, water vapor and nitrogen, all become part of the kiln atmosphere and depending upon the type fuel used and the composition of the concrete articles, the concentration of carbon dioxide may prove detrimental. For example, carbonation of the concrete caused by a chemical reaction of the carbon dioxide with the calcium hydroxide freed from the calcium silicates and aluminates in the cement during hydration often leads to formation of a skin of calcium carbonate on the wet surface of the concrete article. If carbonation occurs before the cement hydration compounds are formed and the cement gel is in an unstable condition, the results are detrimental and even though the strength of the cured article may be sufficiently high, the surface and corners thereof will be brittle and lead to excessive breakage in handling.
Another curing technique currently in use is high pressure steam curing using high temperature steam and such methods typically introduce steam into an autoclave type kiln at a temperature range of 350.degree.-365.degree. F and under saturated steam pressures of 120-150 psi. Under these conditions of temperature and pressure, the rate of chemical reaction of the cement is much accelerated as compared to that in the low pressure steam curing technique which typically operates in a temperature range of 140.degree.-180.degree. F. However, in high pressure steam curing, if the steam is allowed to enter the kiln before the concrete articles are allowed to attain their initial set, or if the pressure is brought up too fast, thermal shock may result. This in turn can cause crazing or cracking of the surface, distortion of shape, or formation of brittle articles. Another drawback of this method is the high energy input needed to generate high pressure steam and the relatively expensive cost of installing and operating such a system as compared to low pressure systems.
Another type steam curing technique involves circulating hot oil at a temperature of about 400.degree. F through a network of piping and sprinkling cool water directly onto the heated piping. The water, upon contacting the oil-heated piping, is converted to low pressure steam. This low pressure steam system suffers the same disadvantages and drawbacks as discussed heretofore with respect to the other prior art steam curing systems.